Unlike dry, granular starch, thermoplastic starch (TPS) is capable of flow and hence polymer blending protocols can be applied to such a material. Starch is a polar, hence hydrophilic material.
Since starch is an inexpensive, renewable and biodegradable resource, blends of synthetic polymers and TPS represent a route towards ecologically and economically viable plastics.
However, synthetic polymers are known to be sensitive to TPS loading and their mechanical properties quickly suffer with the presence of TPS. It is therefore key to provide new materials and related methods which, despite TPS loading, maintain or even improve the mechanical properties of the end products when compared to pure (virgin) synthetic polymers.
As used herein, the term “starch” refers to any starch of natural origin whether processed, chemically modified or treated, including starches such as for example: wheat starch, corn starch, potato starch, and rice starch. Starch can also be derived from plant sources such as cassava, tapioca, and pea. It is a polysaccharide that consists essentially of a blend of amylose and amylopectin.
Starch includes modified starches, such as chemically treated and cross-linked starches, and starches in which the hydroxyl groups have been substituted with organic acids, to provide esters or with organic alcohols to provide ethers, with degrees of substitution in the range 0-3.
Starch also includes extended starches, such as those extended with proteins; for example with soya protein.
As used herein, the expression synthetic polymer refers to the materials listed below and mixtures thereof and includes any substantially non-polar hence water-insoluble or hydrophobic synthetic thermoplastic or thermoset material. Examples of substantially water-insoluble thermoplastic homopolymer resins are polyolefins, such as polyethylene (PE), polypropylene (PP), polyisobutylene; vinyl polymers, such as poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVA), poly(vinyl carbazoles); polystyrenes; substantially water-insoluble polyacrylates or polymethacrylates, such as poly(acrylic acid) esters, poly(methacrylic acid) esters; polyacetals (POM); polyamides, such as nylon6, nylon-6,6, aliphatic and aromatic polyamides; polyesters, such as poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT); polyarylethers; polyurethanes, polycarbonates, polyimides, and high molar mass, substantially water-insoluble or crystallizable poly(alkylene oxides), such as poly(ethylene oxide), poly(propylene oxide).
Further included are polyesters and polylactides that are considered biodegradable in short time periods. Examples of those water insoluble materials are polylactones such as poly(epsilon-caprolactone) and copolymers of epsilon-caprolactone with isocyanates; bacterial poly(hydroxyalkanoates) such as poly(hydroxybutyrate-3-hydroxyvalerate); and polylactides such as poly(lactic acid), poly(glycolic acid) and copolymers comprising the repetitive units of both.
Further included are substantially water-insoluble thermoplastic α-olefin copolymers. Examples of such copolymers are alkylene/vinyl ester-copolymers such as ethylene/vinyl acetate-copolymers (EVA), ethylene/vinyl alcohol-copolymers (EVAL); alkylene/acrylate or methacrylate-copolymers preferably ethylene/acrylic acid-copolymers (EAA), ethylene/ethyl acrylate-copolymers (EEA), ethylene/methyl acrylate-copolymers (EMA).
Further included are styrenic copolymers, which comprise random, block, graft or core-shell architectures. Examples of such styrenic copolymers are α-olefin/styrene-copolymers preferably hydrogenated and non-hydrogenated styrene/ethylene-butylene/styrene copolymers (SEBS), styrene/ethylene-butadiene copolymers (SEB); styrene acrylonitrile copolymers (SAN), acrylonitrile/butadiene/styrene copolymers (ABS).
Further included are other copolymers such as acrylic acid ester/acrylonitrile copolymers, acrylamide/acrylonitrile copolymers, block copolymers of amide-esters, block copolymers of urethane-ethers, block copolymers of urethane-esters.
Further included are thermoset resins such as epoxy, polyurethane and polyesters.
It is known to prepare immiscible blends of TPS and polyolefin materials. Since one material is hydrophobic while the other is hydrophilic, the materials tend to form distinct domains or “islands”. Large islands are not desirable in most applications since these regions do not have the mechanical properties of the polyolefin.
U.S. Pat. No. 6,605,657 (U.S. Pat. No. '657) teaches making a blend of TPS and a polyolefin, such as polyethylene. The disclosure of U.S. Pat. No. '657 is incorporated herein by reference thereto. The materials obtained in accordance with U.S. Pat. No. '657 typically contain from 50 to 60 weight percent of TPS and yet maintain good mechanical properties.
Description of the Method of Preparing the Novel Compositions
The method of the present invention uses a starch suspension as a first feed material and a synthetic polymer as a second feed material. The synthetic polymer is preferably ground into granules for ease of melt processing through a screw-type blender-extruder.
Referring now to FIGS. 12 and 13, there is shown a preferred embodiment of the extrusion apparatus used to carry-out the method of the invention. Referring to FIG. 12a, an upper view of the extrusion system 10 shows a twin-screw extruder (TSE) 12 to which is attached a single-screw extruder (SSE) 14. In sharp contrast with the prior art, the thermoplastic starch (TPS) is prepared in the TSE 12 while the synthetic polymer, in this case low density polyethylene (LDPE), is melted in SSE 14. This method will be further described hereinbelow.
Preparation of the Starch Suspension
Wheat starch was mixed in different proportions with water and glycerol. During the starch extrusion, water is important to promote the gelatinization process. Once gelatinized, the glycerol plasticizes starch. In addition to plasticizing starch; glycerol decreases the viscosity of TPS. In the suspension, the starch content varied from 48 to 50% by weight. Water and glycerol were varied from 20% to 30% and from 32% to 19% by weight, respectively. The glycerol concentration was varied in order to achieve TPS of varying and controllable viscosities. The water content was modified to maintain a constant liquid/solid ratio of about 1:1 v/v. Three examples are reported in Table 10 below. All contents are expressed in terms of %/wt of suspension.
TABLE 10Starch GlycerolWaterExamplecontent*content*content*1483220248.527.5243502030*In the initial slurry
In a typical suspension, 640 g of glycerol was mixed with 400 g of distilled water and placed in a recipient. 960 g of starch powder was poured in the recipient containing water and glycerol and stirred to give a homogeneous slurry. The slurry, once made, was used immediately in the preparation of LDPE/TPS blends. Starch suspensions are susceptible to the problem of sedimentation. Furthermore, the viscosity of the starch suspension increases with time. This increase has been attributed to the solvation of starch molecules and further re-arrangement into a gel-like structure. For these reasons the starch suspension must be used as fresh as possible, especially if the viscosity affects the feeding rate.
Feeding the mixture to the extruder as a slurry is a novel approach to preparing these materials and ensures that the starch is fully destructurized and that the glycerol is well dispersed throughout the starch material. Both of those elements are necessary components to achieving blends with the high elongational properties achieved by the present invention.
One-Step Extrusion Process
a) Basic Setup
Blending was carried out in a one-step process. A single-screw extruder (SSE) 14 was connected to an intermediate zone of a co-rotating twin-screw extruder (TSE) 12 using a leak-proof adapter. The schematic representation of the upper and side views of the extrusion system are showed in FIGS. 12 and 13, respectively. This one-step approach allows for the melt-melt mixing of the components which improves the morphology control of the dispersed TPS phase. It also provides the possibility of minimizing the contact time between the two polymers at high temperature which is the principal parameter for controlling the thermal degradation of TPS. The single screw used was from C.W. Brabender Instruments (L/D=26, length=495 mm, and compression ratio=2). The twin-screw was a Leistritz AG (LM 30.34), LID=28, and length=960 mm. The above described setup allows for the separation of the different processes occurring in this operation. Accordingly, the melting of LDPE takes place in SSE 14, while both the starch gelatinization and plasticization (SGP) and melt blending occur in TSE 12. The mixing of TPS and PE occurs in the latter half of TSE 12. For ease of description, TSE 12 is pictorially divided into zones 16 to 30 as the blending progresses.
b) TPS Preparation
An important feature of the present method is the preparation of the TPS which comprises the steps of starch gelatinization and plasticization (SGP). The screw configuration in TSE 12 was chosen to give a long enough residence time, which permits SGP in the first zones of TSE 12. SGP took place over three sub-sections of TSE 12: feeding section 16, SGP sections 18 and 20 and water extraction section 22. The starch suspension was fed at a temperature lower than 25° C. in the first section of TSE 12. This zone was water-cooled in order to maintain a low temperature. SGP was carried out in the sections 18 and 20 of the TSE 12. Due to the thermal instability of starch, SGP was carried out at 70° and 90° C. in the sections 18 and 20, respectively. Several kneading sections were used to homogenize the resulting TPS. Back-flow kneading elements were also adapted to increase the residence time and, consequently, ensure the complete destructuring and the homogeneity of the TPS. It also served to increase the extrusion pressure before the venting zone 22. Water extraction took place in section 22 of TSE 12. Low-pressure elements, a higher temperature (110° C.) and vacuum were found to improve the water extraction. The venting zone 22 was connected to a condensation system, which avoided the passage of volatiles through the vacuum line. Once the TPS is substantially water-free, it can be blended with the second polymer, in this case LDPE.
The flow rate of the extruded TPS had an influence on the pressure exerted by the starch and its final appearance. In order to study this phenomenon, an TSE extruder configuration using just five zones was used. This configuration was similar to the original eight zones configuration, but zones 24, 26, 28 and 30 were taken out. Three capillary dies were used to measure the viscosity of TPS. The flow rate of the starch suspension was compared to that of TPS at the exit of the capillary die. Surprisingly, the difference between both flow rates was almost equal to the water content in the starch suspension. Likewise, TGA measurements indicated that the water content in TPS was around 1%. This approach is thus very effective in removing the water from thermoplastic starch. This is a critical point since excess water gives rise to bubbles in the resulting starch/polymer blend. These bubbles not only affect aesthetics but also diminish the mechanical properties of the blend. As such, TPS will be considered as a binary system composed of starch and glycerol.
In studying the effect of flow rate of the starch suspension on the quality of the extrudate, lower and upper limits of feeding were found. The lower limit was imposed by the increased residence time of the TPS. It is well known that the TSE works better under starve-fed conditions. In such a situation, the residence time is controlled by the screw configuration, the flow rate and the screw speed. The screw speed was maintained constant at 150-rpm in the whole series of melt mixing and viscosity measurement experiments. Evidence of degradation was found at flow rates of the extruded TPS lower than 20 g/min. The appearance of TPS changed from a transparent and flexible material to a yellowish more rigid one. When the flow rate of TPS was lower than the mentioned limit, an unexpected increase in the pressure was also monitored. At higher flow rates, the pressure was proportional to the measured flow rate. The upper limit for the flow rate of TPS was imposed by the water extraction in the venting zone 22. Problems of foaming were observed at flow rates between 45-50 g/min of TPS. In contrast to the lower limit, the pressure exerted by the foamed TPS decreased as the flow rate increased. Both phenomena were produced by the presence of water in the extrudate. Water vapor, at 150° C. was responsible for the foaming of TPS. Moreover, water excess reduced the viscosity of TPS in the extruder. This upper limit can be overcome by the addition of another venting zone or the modification of the existing one with more efficient equipment. As is mentioned above, the flow rate, temperature, and screw design are important parameters to control.
c) Mixing
The blend mixing section can be divided into three sub-sections: LDPE addition zone 24 mixing zone 26 and 28 and pumping zone 30. The temperature of the whole mixing section was maintained constant at 150° C. As observed in FIG. 12a, the LDPE addition zone 24 has no heating element, however, the temperature was maintained around 150° C. by the convection heating of the neighboring zones 14 and 26 and the molten LDPE. The melt mixing of LDPE and TPS starts in zone 24. The melt mixing continued through the next two zones 26 and 28 aided by several kneading and mixing elements. The pumping zone 30 is necessary to pressurize the extrudate through the die head.
The proportion of thermoplastic starch in terms of wt % of the resulting TPS/polymer blend was about 10 to 60 wt %, and preferably about 20 to 55 wt %.
It is to be noted that by attaching the single-screw extruder 14 progressively downstream (zones 26, 28 or 30) on the twin-screw 12 it is possible to achieve the same level of morphology control as reported here at very low blend residence times. Thus, one of the advantages of the single step approach is that it can be used to minimize the residence time of starch in contact with a high melting polymer. Therefore, TPS can be blended with high melting temperature polymers such as PP, PS, PET etc. while still minimizing thermal degradation of the starch.
The die head 32 and SSE 14 were operated at the same temperature as the mixing section. The screw speed of SSE 14 was kept constant using an arbitrary measure of the motor speed (2.5) and the flow rate of LDPE was controlled with the aid of a pellet feeder. Maximum pumping of SSE 14 under these conditions was 100 g/min.
d) Sheet Take-Up
LDPE/TPS blends were extruded through a rectangular die. Blends were quenched using calendar rolls. Calendar rolls were used because blends could not be quenched in cold water due to the highly hydrophilic nature of TPS. The strain ratio, the ratio between the speed of extrudate and the speed of the ribbon at the exit of the calendar, was around 2. That imposed a machine direction deformation on the ribbon. The morphology of those blends showed evidence of that deformation.
U.S. Pat. No. '657 teaches a one-step extrusion process to obtain a material having a highly continuous TPS phase or even a fully co-continuous blend of TPS and polyolefins while maintaining satisfactory mechanical properties. In general terms, a starch suspension is prepared using predetermined ratios of starch, water and a plasticizer such as glycerol. The extrusion system is composed of a single-screw extruder connected to a twin-screw extruder. The twin-screw extruder is divided into two parts. The first part is used to gelatinize and plasticize the starch suspension. The second part is used to vent-off the volatiles including water-vapor and receive input from the single-screw extruder fed with molten synthetic polymer. The resulting blend contains TPS and synthetic polymer, is essentially water-free and may be further processed using conventional equipment. The detailed method of making in the disclosure portion of U.S. Pat. No. 6,605,657 is incorporated herein by reference.
The material produced in accordance with U.S. Pat. No. 6,605,657 may be conveniently granulated and cooled in the form of pellets for later use as per the present invention. However, the material produced in accordance with U.S. Pat. No. '657 may also remain molten for direct processing and use as per the present invention.
It was heretofore thought that TPS was simply useful to provide more ecologically viable and less expensive polymer blends. However, it was heretofore neither predictable nor predicted that TPS-containing materials processed in the manner of the present invention could actually become new materials by virtue of physico-chemical modifications and, as new materials, essentially maintain or even improve key mechanical properties over non TPS-containing materials.